There are known methods for making automobile exhaust gas purification catalysts and catalysts, first and foremost, and carriers such as porous oxides with heat resistance and large surface areas carry catalyst particles or catalyst auxiliary particles and the like. To secure a large reactive surface area so as to increase the reaction efficiency as a catalyst etc., catalyst particles or catalyst auxiliary particles and the like are preferably carried highly dispersed as fine nano-level particles.
For example, Japanese Patent Publication (A) No. 2001-9279 discloses an NOx storage reduction catalyst using a carrier comprising an alumina particles coated on their surfaces with titania particles as catalyst auxiliary particles (for restoration from S poisoning) of a particle size of 10 nm or less. Further, as a method of production of the same, it is disclosed to set the pH of a slurry comprised of alumina particles and titania sol to less than 5, then afterward raise the pH so as to coat the surface of the alumina particles with titania fine particles. However, with this method, as shown in FIG. 10 of the same publication, in the process of the rise in pH, the titania sol passes through the isoelectric point, so aggregates and immediately coarsens at that point. At the pH at this time, the alumina is charged and does not aggregate together with the titania sol. Further, as the pH is raised, the titania sol aggregate begins to be charged negatively. However, redispersion takes a long time in comparison to aggregation, so the titania aggregates are adsorbed on the alumina particles while still in the aggregated coarsened state. In such a way, in the method of production of Japanese Patent Publication (A) No. 2001-9279, it is very likely that the surfaces of the alumina particles will not be coated by titania fine particles, but that the alumina particles will simply be mixed with the aggregated titania particles.
Also, Japanese Patent Publication (A) No. 2004-331444 discloses a method of production of an exhaust gas purification catalyst comprising adding an aqueous solution of a titania-containing water soluble organic compound to a slurry containing alumina particles, drying it, and firing it to make the alumina surface carry titania. However, with such a solution reaction, an alumina-titania composite oxide forms, the heat resistance drops, the surface area as a carrier ends up falling, and titania nanoparticles cannot be kept carried at a high dispersion.
Further, Japanese Patent Publication (A) No. 2006-272250 discloses a method of aligning particles by preparing two or more types of colloids covered by a carboxyl group-containing polymer or an amine group-containing polymer or other protective material and utilizing the collection of the different colloids due to the affinity acting between the protective materials. However, in this method, it is difficult to definitely prevent aggregation between the colloids, so a state in which nanoparticles are carried in high dispersion cannot be obtained with a stability.
As opposed to this, the Applicants disclosed in WO 2009/001962 a method of making alumina adsorb NH4+ ions and making superhydrophilic titania, on which OH− ions are adsorbed, be adsorbed/carried on the alumina through the potential difference. This thereby allows the alumina particles to carry titania nanoparticles on their surfaces in a high dispersion.
That is, in the above method, both the alumina and the titania are charged negatively in a weak basic solution, but the NH4+ ions are selectively adsorbed on only the alumina and are not adsorbed much at all on the titania, so a potential difference is generated between the alumina and titania. This potential difference causes titania nanoparticles to be adsorbed and carried on the alumina surface. This selective adsorption type carrying mechanism is made possible by the special condition that the titania is superhydrophilic, so the OH− ion adsorption is strong and the NH4+ ion adsorption is weak.
Therefore, the above method is a method which becomes first possible when the carried titania is superhydrophilic and cannot be applied to other particles.
Thus, a high dispersion carrying method that can be applied to nanoparticles other than titania had been sought.